1. Field of the Invention
The invention relates to negative dispersion optical fiber. More particularly, the invention relates to negative dispersion, negative dispersion slope optical fiber having improved transmission characteristics including improved dispersion slope compensation.
2. Description of the Related Art
Optical fibers are thin strands of glass or plastic capable of transmitting optical signals, containing relatively large amounts of information, over long distances and with relatively low attenuation. Typically, optical fibers are made by heating and drawing a portion of an optical preform comprising a refractive core region surrounded by a protective cladding region made of glass or other suitable material. Optical fibers drawn from the preform typically are protected further by one or more coatings applied to the cladding region.
In an effort to improve transmission capacity of optical fibers, wavelength division multiplexing (WDM) systems are used. In general, WDM systems multiplex a plurality of information channels onto a single fiber, with each channel operating at a different wavelength. Typically, WDM system arrangements include a positive dispersion fiber (PDF) concatenated with a negative dispersion fiber such as a dispersion compensating fiber (DCF) or an inverse dispersion fiber (IDF). The positive dispersion fiber typically comprises a single mode fiber designed to introduce dispersion to reduce the nonlinear interactions between channels. The negative dispersion fibers have negative dispersion to compensate for the linear effects of accumulated dispersion. Typically, dispersion compensating fibers are used on spools in discrete modules that are concatenated with the positive dispersion fiber. Thus, dispersion compensating fibers do not contribute to the transmission distance but still add discrete loss and polarization mode dispersion to the system. For cabling operations, where fiber used to compensate the dispersion of the positive dispersion fiber is deployed in the cable and thus contributes to the transmission distance, inverse dispersion fiber is used.
It would be desirable to have negative dispersion fiber such as an inverse dispersion fiber with improved characteristics such as increased effective area, e.g., for reduction of nonlinear impairments therein, and improved dispersion slope compensation of positive dispersion optical fibers, e.g., for broadband dispersion compensation.
The invention is embodied in an optical communications system including one or more optical transmission devices, one or more optical receiving devices, and at least one optical fiber cable coupled therebetween that includes at least one positive dispersion optical fiber and corresponding inverse dispersion optical fiber. According to embodiments of the invention, the inverse dispersion fiber has negative dispersion and a negative dispersion slope. The inverse dispersion fiber includes a doped core region with an index of refraction n1, a cladding region with an index of refraction n2, and a trench region, a first barrier region and a second barrier region with indices of refraction n3, n4, and n5, respectively, formed between the doped core region and the cladding region. Inverse dispersion fiber according to embodiments of the invention has a chromatic dispersion of approximately xe2x88x9240 picosecond/(nanometer-kilometer) and a relatively large effective core area, Aeff, e.g., greater than approximately 30.0 xcexcm2, both at a wavelength of 1550 nanometers. The various regions of the inverse dispersion fiber are manufactured in such a way that the refractive index value ranges are, e.g., 0.745% less than (n1xe2x88x92n2)/n2 less than 0.759%, xe2x88x920.403% less than (n3xe2x88x92n2)/n2 less than xe2x88x920.394%, 0.152% less than (n4xe2x88x92n2)/n2 less than 0.166%, and xe2x88x920.083% less than (n5xe2x88x92n2)/n2 less than xe2x88x920.041%. Manufacture of the optical fiber includes manufacture of the core region having a diameter of approximately 4.36 xcexcm, the trench region having a diameter of approximately 10.92 xcexcm, and the first barrier region having a diameter of approximately 18.28 xcexcm. The core region is doped, e.g., with germanium or other suitable material. The trench region, the first barrier region, the second barrier and the cladding region are doped, e.g., with germanium and/or fluorine, and/or other suitable material(s). Inverse dispersion optical fiber according to embodiments of the invention provides improved compensation of positive dispersion optical fibers, including existing positive dispersion optical fibers.